Clinto, If I understand the OP right he's talking about the engine (not the turbo).
If you are in fact talking about the engine, if the data plate isn't there I don't know how you tell for sure. Most of the "C" engines I've seen have a different flame heater than the "D" but that's not always the case. I have seen some "C" engines with the newer flame heater. I'd say if you have the older style flame heater it's a pretty safe bet it's a "C" though.
On a side note, was there an "A" and "B" engine? All I've ever seen is "C" and "D".